The present invention relates to I/O pins that connect to both digital and analog circuitry. In particular, it relates to an analog transfer gate with over-voltage protection.
Many types of protection circuits are used for the I/O pins of an integrated circuit. Many of these circuits deal with electrostatic discharge (ESD) protection. An example is U.S. Pat. No. 6,424,510. These circuits are designed to protect against very high voltages, typically in the form of a spike, which might be applied to a pin.
Another type of protection circuit is required for normal operation of a chip. In recent years, the voltage supply for chips has been reduced from 5 volts to 3.3 volts to 2.5 volts or lower. Accordingly, a particular chip does not know if it will be interfacing with a chip of the same voltage, or a higher voltage. For example, a 3.3 volt chip may receive a 5 volt digital signal. For digital signals, the input buffer can simply translate the 5 volt into a 3.3 volt or other level signal.
When an analog circuit is connected to the same pin as a digital circuit, the 5 volt signal which simply needs to be converted for the digital circuit can cause problems for the analog circuitry. For this reason, typically separate pins are used for analog and digital I/O signals. However, it would be desirable to be able to use the same pin for both analog and digital internal circuits, while providing over-voltage protection of the analog circuit from higher digital signals.